In recent years, in the field of color display devices used for computers and TVs to display images, a display device employing a plasma display panel (hereinafter also referred to as PDP) has received attention because such a device enables a large size, low profile and light-weight color display device. A plasma display device utilizing a PDP performs full color displaying by conducting additive color mixture of so-called three primary colors (red, green and blue). In order to perform such full color displaying, a plasma display device is provided with phosphor layers that emit light in respective colors including red (R), green (G) and blue (B) as the three primary colors, and phosphor particles making up these phosphor layers are excited by ultraviolet rays generated in a discharge cell of a PDP so as to generate the respective colors of visible light.
As compounds used for phosphors in the respective colors, (YGd)BO3:Eu3+ and Y2O3:Eu3+ that emit red, Zn2SiO4:Mn2+ that emits green and BaMgAl10O17:Eu2+ that emits blue are known, for example. These phosphors are manufactured by mixing of prescribed raw materials, followed by baking at high temperatures of 1,000° C. or higher so as to initiate a solid phase reaction (See Phosphors Handbook, pages 219 and 225, published by Ohmsha, for example). These phosphor particles obtained by baking are ground and screened out (the average particle diameter of red and green: 2 μm to 5 μm and the average particle diameter of blue: 3 μm to 10 μm) for the use.
The reasons for grinding and screening out (classifying) the phosphor particles are as follows: when a phosphor layer is formed on a PDP, a general technique adopted is to screen-print pastes of phosphor particles in respective colors. When applying the pastes, phosphors with smaller and more uniform particle diameters (i.e., more uniform particle size distribution) allow a better coated surface to be attained easily. That is, phosphors with smaller and more uniform particle diameters having a shape closer to a sphere enable a better coated surface; the enhancement of the filling density of phosphor particles in a phosphor layer; and an increase in light-emitting surface area of the particles. Furthermore, such phosphors can alleviate instability during address driving. Theoretically, it can be considered that this results from an increase in brightness of a plasma display device.
However, when the particle diameters of the phosphor particles are decreased, the surface area of the phosphors would increase or defects on the surface of the phosphors would increase. Therefore, the surface of the phosphors tends to attract a large amount of water, carbonic acid gas or hydrocarbon based organic substances. Especially, in the case of a blue phosphor made of Ba1-xMgAl10O17:Eux or Ba1-x-ySryMgAl10O17:Eux (where 0.03≦X≦0.20, 0.1≦Y≦0.5) that is an alkaline-earth metal aluminate phosphor containing divalent europium (Eu) as an activator, its crystal structure has a layer structure (See Display and Imaging, 1999. Vol. 7, pp 225 to 234, for example), and oxygen (O) in the vicinity of a layer containing Ba atoms (Ba—O layer) among these layers has defects (See Applied Physics, vol. 70, No. 3, 2001, p310, for example).
For that reason, water present in the air selectively is adsorbed to the surface of the Ba—O layer of the phosphor. Therefore, a large amount of the water is ejected in a panel during the course of a panel manufacturing process, and problems occur, such as deterioration in brightness caused by the reaction with the phosphor and MgO during the discharge (particularly, deterioration in brightness of blue and green), a change in chromaticity of the panel (shift in color caused by the change in chromaticity and burn-in of a screen), a decrease in driving margin and an increase in discharge voltage. Furthermore, when vacuum ultraviolet light (VUV) of 147 nm is adsorbed to the oxygen defects, another problem occurs such that the defects further increase, which further increases deterioration in brightness of the phosphor. To cope with these problems, a method is proposed in which the entire surface of the phosphors is coated with Al2O3 crystals for the purpose of allowing the defects of the conventional Ba—O layer to recover (See JP 2001-55567 A, for example). However, the coating on the entire surface causes the absorption of ultraviolet rays, which results in a problem of a decrease in brightness of light emitted from the phosphors and a problem of a decrease in brightness due to ultraviolet rays.